v2: Add support for the phuslog logging provider

[ogimenezb]

Integrates phuslog into go-grpc-middleware/providers.

I welcome all feedback!

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[ogimenezb]

@googlebot I fixed it.

[johanbrandhorst]

Except for the logging behavior when using an unknown log level, this LGTM.

[johanbrandhorst]

LGTM

[codecov-commenter]

Codecov Report

Merging #425 (ceaff62) into v2 (a79558a) will decrease coverage by 27.56%.
The diff coverage is 40.60%.

@@             Coverage Diff             @@
##               v2     #425       +/-   ##
===========================================
- Coverage   83.58%   56.01%   -27.57%     
===========================================
  Files          30       38        +8     
  Lines         932     1346      +414     
===========================================
- Hits          779      754       -25     
- Misses        114      520      +406     
- Partials       39       72       +33     

Impacted Files	Coverage Δ
chain.go	0.00% <ø> (-90.91%)	⬇️
interceptors/auth/auth.go	100.00% <ø> (ø)
interceptors/auth/metadata.go	100.00% <ø> (ø)
interceptors/logging/options.go	82.60% <0.00%> (-8.02%)	⬇️
interceptors/ratelimit/ratelimit.go	50.00% <0.00%> (-50.00%)	⬇️
interceptors/tags/fieldextractor.go	13.79% <0.00%> (-71.51%)	⬇️
testing/testpb/interceptor_suite.go	0.00% <0.00%> (ø)
testing/testpb/test.manual_extractfields.pb.go	0.00% <0.00%> (ø)
testing/testpb/test.manual_validator.pb.go	0.00% <0.00%> (ø)
testing/testpb/test.pb.go	36.92% <36.92%> (ø)
... and 51 more

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